In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, efficient use of the available resources is needed in order to provide ubiquitous high data-rate coverage. Using multiple antennas at the transmitter and/or at the receiver could make it possible to exploit the spatial degrees of freedom offered by multipath fading inside the wireless channel in in a wireless communications network in order to provide high data rates and reliability of wireless transmission. In general terms, in the downlink (i.e., transmission from network to wireless device), there are three basic approaches for utilizing the antenna: diversity, multiplexing, and beamforming. With beamforming, the radiation pattern of the antennas may be controlled by transmitting a signal from a plurality of antenna elements with an antenna element specific gain and phase. In this way, radiation patterns with different pointing directions and beam widths in both elevation and azimuth directions may be created.
Existing so-called macro antennas comprises multiple radiating antenna elements. These antenna elements can be passively combined in order to generate a narrow transmission beam with high gain to supply coverage at cell borders of large cells. Some antenna arrays have the possibility to change the phase between the individual antenna elements in order to change the tilt of the antenna beam pattern, referred to as remote electrical tilt (RET). This change of tilt is however relatively slow, and is only performed to generate a beam pattern that is good for all, or at least the majority, of the wireless devices within the cell. Optimizing the tilt (cell shape) for the intended wireless devices in the cell may be cumbersome, since the radio access network node providing network access in the cell does not know where the wireless devices are located in the cell. Not being able to measure where the signals from the wireless devices come from results in such optimization methods becoming blind. State of the art mechanisms for optimizing the cell specific beamforming, so called reconfigurable antenna system (RAS) self-organizing network (SON) algorithms, hence utilize trial and error methods wherein different settings are evaluated for a period of time, and the best setting is chosen. Such procedures may then be run continuously in different parts of the communications network, so as to iteratively improve the network settings. One issue with such mechanisms is that they are slow, and may temporarily decrease network performance when evaluating non-optimal tilt-settings.
Using active antenna arrays, with individual access to each radiating antenna element, makes it possible to estimate spatial channel characteristics, such as direction of arrival (DoA) to each wireless device, angular spread etc. By utilizing knowledge of the propagation channel as well as the structure of the antenna array it is possible to form narrow beams to each wireless device and thus obtain high received power for each wireless device, so called device specific beamforming. Radiating the majority of the energy in/towards the intended wireless device (or wireless devices), whilst minimizing the radiated power elsewhere not only increases the received power for the intended wireless device (or wireless devices) but also decreases the interference generated.
Using active antenna arrays with one radio per antenna element may however be costly. The antenna array may therefore be split into multiple antenna subarrays, where each antenna subarray is connected to a transceiver unit, with a feeding network. In certain implementations, each subarray is fed by one network and the feeding network within one antenna subarray is passive. This approach with antenna subarray splitting would maintain the maximum antenna array gain; however, the area where this gain is achievable is limited by the antenna radiation pattern from the subarray, as well as the observability on the measured antenna subarray ports.
Hence, there is still a need for an improved control of an antenna system.